In a simple case, the load capability of the DC voltage source is determined from its open-circuit voltage before it is switched into the grid via the inverter. However, this open-circuit voltage is present when there is no load on the DC voltage source, and therefore, it does not adequately reflect the instantaneous performance of the DC voltage source. For example, if at the time a photovoltaic module is switched into the grid in the morning, the weather conditions are unfavorable, the photovoltaic module may indeed reach the voltage threshold required for it to be switched into the grid, but may still not deliver a power output which, after subtracting the internal consumption of the downstream inverter; i.e., the power required to operate the inverter, would be sufficient to keep the inverter permanently connected to the AC power grid in feed-in mode. As a result, shortly after the inverter is switched into the grid, it is preferably disconnected from the grid again. The resulting additional switching operations considerably reduce the service life, in particular that of the grid disconnect switch.
In one approach, a DC input voltage, which may be the output voltage of the DC voltage source or the voltage of a DC link of the inverter, is measured when the grid disconnect switch is still open but the inverter is already activated, so that the internal consumption of the inverter acts on the DC voltage source as a small test load. The output voltage of the DC voltage source is thus no longer the pure open-circuit voltage thereof. However, the internal consumption of the inverter is generally too low to serve as a test load to determine whether or not the DC voltage source can be switched into the grid. That is, the load capability of the DC voltage source as determined with the aid of the internal consumption still does not adequately represent the load that can be placed on the DC voltage source at the time it is switched into the grid.
German document DE 43 25 436 C2 describes a method including short-circuiting a DC voltage source through the inverter while the grid disconnect switch is still open to thereby place a higher test load on the DC voltage source prior to switching it into the grid. However, this procedure leads to heavy loading of the inverter components carrying the short-circuit current and is in contradiction to the objective of giving all components of the device used a long service life.
U.S. Pat. No. 7,269,036 B2 describes an approach in which the load capability of a DC voltage source is determined only after the DC voltage source has been switched into the AC power grid. The load capability so determined is used as a basis to determine the open-circuit voltage of the DC voltage source that must be reached before the next attempt is made to switch it into the AC power grid. In this manner, it may indeed be possible to determine an optimized value for the open-circuit voltage for the future as an indication of a sufficient load capability of the DC voltage source; however, in this way, only a small number of unsuccessful attempts to connect to the grid and associated stresses on the grid disconnect switch will be preventable as compared with using a fixed voltage threshold for the open-circuit voltage. In addition, according to U.S. Pat. No. 7,269,036 B2, the load capability of the DC voltage source that is already switched into the AC power grid is determined measuring the DC current flowing from the DC voltage source. The measuring device required for this purpose is not present in most of the inverter-based devices to which the present invention is directed.